As an adhesive for use in industries, an acrylic adhesive containing an acrylic copolymer is generally used because it can be easily manufactured at low cost.
An industrial adhesive requires durability in a wide variety of conditions depending on the application thereof. For example, an optical adhesive for bonding an optical film typified by a polarizing plate or a phase difference film to a display such as a liquid crystal panel needs to maintain the performance thereof even under an environment requiring severe conditions, such as in-car displays and outdoor displays, and requires excellent durability under a usage environment.
Particularly, in order to exhibit creep properties under a high-temperature environment (heat creep resistance), there has been proposed an acrylic adhesive composition in which the weight average molecular weight of an acrylic copolymer is set to 1,000,000 or more, and which is used in combination with a crosslinking agent (for example, Patent Document 1).
However, since the acrylic adhesive composition described in Patent Document 1 contains an acrylic copolymer having an extremely high weight average molecular weight of 1,000,000 or more, solution viscosity thereof is high, and thus a large amount of an organic solvent is required to adjust the viscosity to the extent that coating properties become excellent. Further, since the acrylic adhesive composition uses a crosslinking agent, the curing of an adhesive is required, and the variation in performance of the adhesive easily occurs due to uneven crosslinkage.
In recent years, an adhesive composition containing an acrylic triblock copolymer has been proposed instead of a crosslinked acrylic adhesive (for example, Patent Document 2).
The adhesive composition described in Patent Document 2 is a non-crosslinked adhesive, and decreases solution viscosity by using a triblock copolymer instead of an acrylic copolymer, so as to improve coating properties. However, since the adhesive composition described in Patent Document 2 has heat resistance up to approximately 90° C., it is necessary to blend a plurality of adhesives in order to exhibit higher heat creep resistance.
Meanwhile, in order to improve heat creep resistance, there has been proposed an adhesive composition in which a silane coupling agent or an isocyanate additive is added to an acrylic triblock copolymer (for example, Patent Document 3).